Conventional Fresnel lenses for use as condensers, concentrators, and field lenses usually are of the form of a single element with prismatic, circular grooves on one side and a planar surface on the other. When high transmission is required at a smaller f-number, for example in overhead projectors, two of this type are used together and are sealed together at the periphery. Since these lenses usually lie close to the imaging plane or stage of the projector, Moire Interference patterns can appear on the projected image when the centers of the circular groove structures are not coincident to a certain tolerance.
A single element Fresnel lens with identical groove structures on both sides is described in Japanese patent No. 57-109618. In that patent, two identical dies are replicated from the same groove structure and are very accurately aligned in a fixture using guide posts. The lens is formed by pouring a thermoplastic resin between the dies and subsequently applying heat and pressure.
Such a dual-element lens is economically advantageous over the conventional two lens arrangement since only one sheet of lens material is necessary. Unfortunately, the requirement that the center of the grooved surfaces be aligned to a close tolerance increases the expense of a workable dual-element lens, and therefore somewhat offsets its advantages.
It is desired to reduce the cost of manufacturing a workable dual-element lens by eliminating or easing the requirement for highly accurate center alignment of the grooved elements on each side of the lens.